The invention relates to an endoprosthesis for a shoulder joint as generically defined by the preamble to claim 1.
From European Patent Disclosure EP 0 712 617, a shoulder joint prosthesis is known whose shaft part has an indentation with a hemispherical bottom in its metaphysial end, into which indentation a ball is introduced. This ball has a central continuous bore and a slot that communicates with the bore over its entire length and splits the ball ring, making it C-shaped. A stem connected to the cap is inserted into the bore. Grub screws are provided in the shaft part, in order to immobilize the ball in the indentation in a selectable angular position and to deform the ball ring and thereby firmly clamp the stem in it.
A disadvantage of this prosthesis is that the cap must be precisely placed in every direction before it can be fixed in this position. To that end, it must be held in this position by a surgeon while the grub screws are being tightened forcefully. Furthermore, the optimal position of the cap can be only approximated, since the spacing between the spherical cap axis and the shaft axis is constant. An alignment of the cap center in the anterior/posterior direction always necessarily involves a displacement in the cranial or caudal direction. Furthermore, the grub screws must be tightened before the prosthesis is inserted, because afterward they are no longer accessible. This requires multiple test manipulations of the arm being operated on and corrections to the cut face of the bone during the operation and makes a correction of the alignment after implantation has been completed impossible.
French Patent Disclosure FR 2 773 469 discloses a shoulder joint prosthesis with a shaft part and on it a pivotable adapter or directional piece, and the adapter has a ball cap. In this prosthesis, a hemispherical pit with a threaded bore on its bottom is provided on the metaphysial end of the shaft part. The pit and the threaded bore are aligned with a neck axis, whose angle to the shaft axis is selected in advance. Seated in the pit is a pivotable adapter, with a hemispherical surface, concentric with the hemispherical surface of the pit, on its humeral end. On its other end, the adapter has a conical surface, which is eccentric with regard to the center of the sphere common to the two spherical surfaces, on which conical surface the spherical cap is seated with a suitable conical indentation. The axis of this indentation is eccentric to the center point of the cap. The adapter has an axial bore with a hemispherical bottom. A screw with a ball head is introduced, with the shaft leading, into the bore, and the threaded portion of the shaft is punched through a conical bore in the hemispherical bottom of the adapter and screwed into the threaded bore in the shaft part. The centers of the spherical surfaces of the pit in the shaft part, the inner and outer hemispherical surface on the adapter, and the spherical surface of the screw head are located at a common central point, once the screw has been tightened. As a result, the adapter is pivotable relative to the shaft part in all directions and simultaneously rotatable. Pivoting the adapter causes its longitudinal axis to shift parallel to the desired neck axis direction, and rotating the adapter about its longitudinal axis and rotating the cap in a manner adapted to that rotation makes it possible to compensate for the deviation of the neck axis to be attained from the approximated neck axis. Once the optimal location of the adapter is definite, it can be fixed to the shaft part by tightening the screw.
In this prosthesis as well, the direction of the joint neck is adjustable infinitely variably, depending on the given conditions of the original joint. By adjusting the inclination and rotation relative to the shaft axis with the aid of a manipulation head, the neck axis is adjustable parallel to the optimal neck axis. This approximation can then be corrected infinitely variably afterward transversely to the direction of the axis with the cooperating eccentrically disposed conical surfaces on the adapter and the cap. The length of the joint neck is regulated by a set of caps of different thicknesses. In this way, a spherical cap can be made firm at any arbitrary site in any desired position, so that the site of the cap can be oriented optimally to the original conditions of the joint or the intended correction and the original center of rotation of the shoulder joint can be restored.
Since the success of a shoulder joint depends essentially on the restoration of the muscular equilibrium of the shoulder musculature, it is important that the original center of rotation of the shoulder joint be restorable by means of this individual placeability of the spherical cap. The closest possible spacing between the cut face of the bone and the artificial cap is also attainable, so that in every motion, the muscles are offered a continuous support point. It is also advantageous that the inclination and rotation of the adapter relative to the shaft part can be oriented to the cut face of the bone or to the desired orientation of the cap and can be fixed in that position even after the implantation of the shaft part, since the screw that fixes the adapter remains accessible from the side of the cap.
A disadvantage of this prosthesis, however, is that the spherical surfaces must be produced exactly concentrically, which is complicated and expensive. Ball joints furthermore offer a very small support face. In addition, the support face is reduced in size, in the region of the main direction for tightening the screw, by the conical bore in the spherical wall of the adapter. Ball joints in general have the disadvantage of not allowing a press fit in the sense of a conical clamping action. Therefore even after the operation is concluded, the fixation of this ball joint remains insecure.
Furthermore, the adapter must be placed correctly in every respect before it can be allowed to be fixed. In particular, a rotation about the directional axis, namely its own longitudinal axis, in order to rotate the eccentric cone into the desired position, is possible only if the angular position of the adapter is also unstable. Unintentional shifting of the adapter can occur upon removal of the manipulation head, insertion of the screw, tightening of the screw, and adjustment and placement of the spherical cap.
Finding the correct rotary position of the adapter in which the two eccentric conical surfaces of the adapter and spherical cap cooperate in such a way that the spherical cap comes to rest at the desired point is quite difficult. In practice, the eccentricity of the conical surface on the adapter is therefore oriented with the aid of a manipulation head in such a way that the cone axis is located as close as possible to the axis estimated to be optimal, and after that the cap is put in place. Upon alignment of the center of the cap in the anterior/posterior direction, however, there is necessarily also a shift in the cranial or caudal direction.
It is therefore the object of the invention to propose an endoprosthesis for a shoulder joint in which the advantages of the prior art cited are preserved, while the aforementioned disadvantages of ball joints are avoided. In particular, the greatest possible forces of friction should be attained between the individual parts that are displaceable and rotatable counter to one another. Moreover, it should be possible to define the direction and position of the joint axis independently of one another. Compared to the prior art, the calibration of direction and location of the spherical cap axis should be simplified. The sensitivity of the settings of the axial direction, in terms of inclination and rotation, to unintended changes in the inclination position or rotation position should be minimized as much as possible by adjustment of the other setting. Also, the direction of the axis should have the slightest possibly vulnerability to influence from a change in the location of the spherical cap axis relative to the location of the longitudinal axis of the adapter or the directional piece as possible.
This object is attained by disposing a rotating piece between the shaft part and the directional piece, the rotating piece being rotatable relative to the shaft part and a first axis, and the directional piece is supported rotatably relative to the rotating piece about a second axis, and this second axis extends transversely to the first axis and transversely to the directional axis; contact faces between the shaft part and the rotating piece, on the one hand, and between the rotating piece and the directional piece on the other each allow only a relative motion between the shaft part and rotating piece, and the rotating piece and directional piece, respectively, about the respective common first and second axis, as a center of rotation.
As a result of this separation of the pivoting motion of the directional piece into two pivoting motions about two axes that cross one another, the full range of motion that a ball joint offers is preserved, yet in comparison to a ball joint the risk of unintended change of a pivoted position relative to the one axis is markedly reduced by a pivoting about the other axis. The articulation and friction faces between the shaft part, rotating piece and directional piece are not spherical surfaces but rather surfaces of axial bodies of rotation, such as a cylinder, torus, or cone, to name only the simplest ones, whose surfaces are simpler to produce precisely. The contact faces can therefore be embodied with a larger area than spherical surfaces. A press fit is also possible, because conical portions can be provided in the articulation face. The alignment of the first axis around which the rotation piece is rotatable relative to the shaft part can be parallel to a neck axis of the natural joint. It is also possible to have the first axis approximately perpendicular to the neck axis or to the directional axis, and the second axis approximately perpendicular to these two axes. It is also possible to align the first axis with a directional deviation from the direction of the neck axis. This deviation is advantageously less than 60 degrees and preferably less than 50 degrees. Then the second axis is placed crossing the first. As a consequence, both the rotational position and the inclination position can be set. Advantageously, the direction of the first axis is selected to be approximately parallel to or even better in the same plane as that passing through the shaft axis and neck axis. Upon pivoting or rotation of a part of the prosthesis about an axis that is parallel to or located in this plane, the inclination is not adjusted. Only the alignment of the second axis that crosses the first axis is adjusted. As a result, the adjusting direction, dictated by the second axis, can be set. In particular, an alignment of the first axis parallel to the neck axis is expedient.
If the first axis intersects the neck axis, or if the two coincide, then pivoting of the rotating piece about this first axis causes practically no elongation or shortening of the joint neck.
Advantageously, at least one eccentric ring or one oblong slot ring is provided between the directional piece and the spherical cap. This ring makes it possible to vary and define the location of the cap, or the spacing between the directional piece and the center of the cap. This makes it possible to choose the direction and amount of the spacing between the directional piece and the spherical cap axis or the center of the spherical cap. In this way, a displacement of the second axis resulting from pivoting about the first axis can also be compensated for. It is therefore not necessary for the first and second axes to intersect at a point.
In a simplified embodiment, the cap can be placed directly on the directional piece. The indentation with which it is placed on the directional piece can be machined into the cap centrally or eccentrically. Nor does the cap have to have a spherical surface; instead, it can have an embodiment that differs from this idealized articulation face.
Expediently, the directional piece has an external cylindrical or conical surface, whose center of rotation is the longitudinal axis of the directional piece, that is, the directional axis. The spherical cap, eccentric ring or oblong slot ring can be placed on this surface. The conical surface is preferred, on which these parts can be fixed by being pressed onto the directional piece. For the oblong slot ring, which in any case requires a fixation device, such as grub screws, the surface can readily be cylindrical. Cylindrical surfaces in combination with fixation devices, both in the case of an oblong slot ring and in the case of an eccentric ring, make it possible to choose the spacing between this ring and the shaft head, and accordingly the spacing between the center of the spherical cap placed on it on the one hand and the shaft axis on the other.
The alignment of the first axis preferably corresponds to the alignment of a preselected neck axis. As a result, the rotating piece is supported in the shaft part rotatably about the neck axis and can be secured to the shaft part in a manner fixed against relative rotation by very simple means. By adjustment of the directional piece about the second axis relative to the rotating piece, forces which tend to pivot the rotating piece about the first axis need hardly be expected, at least in a narrower region around the middle position, since these two axes and pivoting directions are essentially perpendicular to one another. Since the first axis extends approximately parallel to the directional axis, a fundamentally different manipulation is required for rotation about this first axis compared to that for pivoting about the second axis that is perpendicular to the directional axis.
Advantageously, the rotating piece has a concave rotational body surface oriented toward the directional piece, with the second axis as its center of rotation. Alternatively, however, the rotating piece has a convex rotational body surface oriented toward the directional piece, with the second axis as its center of rotation. In particular, a cylindrical, toroid or conical surface is conceivable, but also more-complicated rotational body surfaces, for instance with steep conical side faces or faces curved in multiple directions, and actual hinge joints with axle bolts placed in them are equally conceivable.
Although the two axes need not intersect, nevertheless in one embodiment the first axis intersects the second axis at a central point. As a result, it is possible to fix the two pivoting motions in common, using a single common fixation device. To that end, both this fixation device, which is preferably a screw that can be screwed into a threaded bore in the shaft part, and the directional piece must have contact faces that rest on spherical surfaces whose centers, in the screwed-together state, coincide with the point of intersection of the two axes, that is, the central point.
Advantageously, there is a recess with a side wall in the shaft part, which side wall is equivalent to a surface of rotation about the first axis or the neck axis. In it, a rotating piece with an outline corresponding to the side wall is supported rotatably about the first axis, and this rotating piece has an articulation face for the directional piece, in the form of a surface of rotation about the second axis. The side wall can be cylindrical or conical or can be provided with a thread. In each case, the rotating piece can be rotated by at least 360 degrees prior to the fixation. Upon fixation, in the case of the conical side wall, a conical clamping to the side wall results, while in the case of the cylindrical side wall, the pressure is exerted via the bottom face of the indentation or of the rotating piece. In the case of a thread, the pressure is exerted over large areas, namely the flanks of the thread.
The directional piece has a shape that is complimentary to the articulation face, toward the directional piece, of the rotating piece and also has an axial conical surface onto which a conical surface of the spherical cap or of an eccentric ring can be slipped. The articulation faces between the directional piece and the rotating piece need not meet over a large area but instead can rest on one another merely linearly or even at a point. For fastening the directional piece to the shaft part, a threaded bore is advantageously provided in the shaft part. The directional piece has an internal bore, in which there is space for the head of a screw, as well as a bore for the screw shaft. With the screw, the directional piece can be screwed into the threaded bore, through a bore in the rotating piece. The bore in the directional piece for the screw shaft is embodied as a slot, which allows a range of motion of the screw shaft of 30 degrees, and whose width is adapted to the diameter of the screw shaft, in particular to the diameter of a threadless part thereof that cooperates with the bore, in order to prevent lateral shifting of the directional piece relative to the rotating piece.
In another embodiment, the two independent rotary motions about the first axis and the second axis are also fixable individually. This makes it easier to lock or fix the individual pivoting motions. As a result, first the rotation of the directional axis relative to the shaft axis can for instance be defined and fixed, and then in a second step the inclination can be determined and fixed.
Where given conditions allow, the spherical cap can have an inner conical surface that instead of being centrally disposed is eccentric relative to the spherical cap, and this inner conical surface can be slipped onto the outer conical surface of the directional piece or of the eccentric ring. As a result, the spacing and direction between the center of the cap and the longitudinal axis of the directional piece can be set. The eccentricity of both the inner conical surface in the cap and the outer conical surface on the eccentric ring is not visible, however, because it is concealed by the cap. This makes the targeted setting of the direction and distance more difficult. For this reason, two eccentric rings that can be put together via conical surfaces are therefore disposed between the spherical cap and the directional piece. They allow visual checking of the location of the cone outlines relative to one another and relative to the outline of the cut face of the bone, so that the eccentric rings can be placed in the center of the outline, so that after that a cap with a central conical surface can be placed on the second eccentric ring.
Advantageously, however, a manipulation head is used instead of a second eccentric ring. The manipulation head has an inner conical surface, disposed eccentrically relative to the spherical cap axis, which can be slipped onto the outer conical surface of the eccentric ring, and whose eccentricity is equivalent to the eccentricity of the inner conical surface of the spherical cap. Furthermore, it is shaped annularly in such a way that the eccentric ring is visible and adjustable through the eccentric annular opening of the manipulation head. In setting the eccentric ring, this makes visual comparison possible between the edge of the manipulation head and the edge of the cut plane of the bone. Because the eccentricities of the cap and manipulation head match, the optimal rotary position of the eccentric ring can easily be fixed by this comparison and the eccentric ring can be firmly hammered or pressed onto the directional piece, and the cap can be placed at the same as the manipulation head, in the same alignment. After that, with an optimally oriented directional piece and eccentric ring, finding the optimal rotary position of the cap is easy.
Alternatively to an eccentric ring between the directional piece and the cap, it is also possible to provide an eccentric part between the rotating piece and the shaft part. The fixation of the eccentric part to the shaft part can be done for instance with a ring screw. The rotating piece and directional piece then need to be secured to the eccentric part.
As an alternative to the double eccentric ring, an oblong slot ring can be provided, which has an oblong slot with which it can be slipped onto the directional piece. The ring has a fixation device, with which it is fixable on the directional piece in a selectable displacement and rotary position relative to the directional piece. The use of an oblong slot ring between the directional piece and the spherical cap is independent of the pivoting device on the shaft part. Accordingly, an oblong slot ring can also be used in a prosthesis in accordance with the known prior art.
In a version of the directional piece that is advantageous for production, its length is located inside a roller or cylinder part having the radius of the convex articulation face. This makes it possible to produce the directional piece from a rolled piece with that radius, by machining the connecting part to the joint head with the internal bore out of the rolled part using metal-cutting means.
In an alternative version to the above, with a screw screwed into the shaft part, a bore that is concentric with the articulation face about the second axis is provided in the rotating piece. This body or head part is connected to a screw shaft, and the wall between the bore and the articulation face has a slot extending transversely to the second axis, through which slot the screw shaft passes. The directional piece has an articulation shim and a pressure shim, and the articulation shim has a bore for the screw shaft and an articulation face corresponding to the articulation face of the rotating piece, and on the opposite side also has a pressure face cooperating with the pressure shim. The pressure shim is connected to the screw shaft. The connection between the pressure shim and the screw shaft assures that by rotation of the pressure body or of the screw shaft, a relative rotation between the screw shaft and the cylindrical body and/or the pressure body results and thereby the distance between the cylindrical body and the pressure shim is varied. To that end, either in the cylindrical body or in the pressure body, or in both, a female thread is provided, which cooperates with a male thread on the screw shaft. By this means the directional piece and the rotating piece can be fixed against one another. The rotating piece is therefore in turn advantageously fixable to the shaft part with a ring that can be screwed onto or into the shaft part, independently of the fixation of the directional piece to the rotating piece.
Advantageously, in such an endoprosthesis for a shoulder joint, at least two of the following parts are put together prior to the implantation of the prosthesis and are available secured at least temporarily to one another: shaft part, rotating piece including an optional head piece, directional piece including an optional articulation shim and pressure shim, and screw, as well as an optional eccentric shim or oblong slot shim. These parts are secured against falling out, for instance with union rings or tongues. This has the advantage that the surgeon need not take small parts, which are hard to manipulate, into his fingers. An expedient combination from among the series of possibilities is to provide the shaft part with the rotating piece secured on it and to provide the directional piece separately, if need be composed of the articulation shim and pressure shim, with a screw inserted or a screw shaft. The rotating piece can also be combined with the directional piece, or all the parts can be temporarily put together.
The cooperating concave and convex contact faces between the rotating piece and the directional piece, between the directional piece and the screw, and between the directional piece and the articulation shim can correspond to one another in geometry and size. However, they can also correspond merely in such a way that a linear contact takes place between the contact faces. To that end, the articulation face on the rotating piece is for instance embodied as a groove, with two flat faces at an angle to one another. The bore in the directional piece into which the screw is inserted correspondingly has a conical bore, rather than a hemispherical hollow, or has one or more circular edges along approximately the same geometric spherical surface. The articulation shim in turn is equipped, like the rotating piece, with for instance two flat faces at an angle to one another. The contact faces can also have edges or spurs that can be pressed into the counterpart face when the screw is tightened. The linear contact has the advantage over area contact that the contact faces are pressed against one another, causing deformation of material, and as a result the connection once the screw is tightened is quite stable. Embodying the concave contact faces in such a way that they touch the convex contact face only along a line furthermore has the advantage that the concave contact faces can be produced very simply and economically.